The use of catheters for site-specific, transcutaneous introduction of diagnostic and therapeutic agents or devices is well known in many medical arts. The many different applications of such catheters sometimes require construction from materials having specialized physical properties and configurations which dictate various construction methods and/or material selection.
Common catheter requirements include the incorporation of a proximal portion that is longitudinally incompressible to enable the catheter to be pushed through a needle or introducer device, or over some type of guiding wire. In this connection, it must have a minimum column strength, either inherently by virtue of the material stiffness of the catheter tubing or from binding together helical wraps in opposing directions, as a result of a reinforcing coil contained within the catheter tubing having initial tension, or imparted to the catheter tubing from a stylet inserted therein. Next, it is desirable to have a highly flexible distal end (i.e., a "soft tip") to minimize the risk of patient injury during catheter insertion and to allow the catheter to advance into tortuous paths. The catheter should have adequate reinforcement to prevent kinking as a result of bending or luminal collapse under radial compression from any fluid path connectors or adapters. Furthermore, the catheter should be designed with as thin a wall as practicable, yet in a manner to maximize tensile strength with minimal tensile compliance to allow controlled withdrawal. It should exhibit radioopacity and be devoid of any long, continuous conductive wires (even simply diamagnetic or paramagnetic materials) contraindicated in MRI procedures.
If a catheter must be manipulated and steered during bodily insertion into relatively long, tortuous paths, it must possess good torsional stiffness along its length and preferably have a relatively lubricious external surface. However, for applications where the insertion path is relatively short, through an epidural needle for example, or where blood flow tends to direct the catheter as in the case of peripherally inserted central venous catheters (PICC catheters), such torsional stiffness is unnecessary. Strong pinching, as can occur with an epidural catheter by the ligamentum flavum and vertebrae, may also place an upper limit on the coefficient of friction which will still allow withdrawal. If the catheter is to be advanced over a guiding wire, an internal surface with a low coefficient of friction is sometimes required. This is achieved by employing either a fluoropolymeric, hydrophillic, or otherwise lubricious coating, or a hard, roughened internal surface that minimizes actual contact area.
When a catheter must enable high rates of injection, the ability to withstand internal pressures of several hundred psi may be required, especially in the proximal region where pressure is highest. Optical transparency may be advantageous in evidencing vascular placement or assuring non-vascular placement. Such transparency is seldom disadvantageous.
The progressive miniaturization of catheters has reached a point in development where the requirements for thin walls, large I.D.'s, good tensile strength, and kink-resistance make using a soft material problematic. There is a need, for example, for a 2 French (2/3 mm O.D.) PICC catheter made from soft silicone or polyurethane. Yet the longitudinal strength of such a small, soft, atraumatic catheter is dangerously low.
Small, soft-tip epidural catheters have even more rigorous requirements. After such a catheter is inserted and the epidural needle is withdrawn over it, a fluid fitting is applied to the catheter's proximal end and achieves a fluid-tight seal by firmly compressing an elastomeric sleeve against the catheter's surface. This compressive force can cause radial collapse. Pinching of the catheter can also occur between vertebrae of an erect spine. The catheter must resist these compressive forces without compromising patency. It must also resist kinking and remain flexible, even when bent sharply.
To enable the catheter to be reliably removed from a tight vertebral pinch without breaking, high tensile strength is required. Despite the best efforts in the industry, breakage of these catheters is an ongoing problem, especially with catheters made of softer, usually tackier materials. The use of a slippery material or coating to reduce the removal force is inappropriate since a lower coefficient of friction also potentiates catheter migration and reduces the force required to remove the compression fitted adapter from the proximal end of the catheter. The reliable function of Tuohy-Borst compression adapters and the like with conventional epidural catheters is already problematic with only a fine line dividing easy removal due to inadequate compression and luminal collapse from excessive tightening.
Although polyamide and hard polyurethane tubing satisfy these requirements, they have a relatively thick wall and, as a consequence, are relatively stiff, causing an unnecessarily high incidence of venous cannulation, paresthesiae, and, in a small number of incidents, permanent nerve damage. In a study conducted by Stephen H. Rolbin, "A comparison of two types of epidural catheters," then recent data from Mount Sinai Hospital, Toronto documented "a 30 per cent incidence of paresthesiae and a 10 per cent incidence of blood returning after insertion of the epidural catheter." Can J Anaesth 1987, Vol 34, No. 5, pp 459-61. The latter problem is especially serious, as inadvertent intravenous injection of the bolus of anesthetic can induce convulsions, hypotension, and loss of consciousness. His paper goes on to report the superior performance of a polyurethane catheter, reported as "much softer than the one made of nylon," which lowered the incidence of paresthesiae from 44 to 24 per cent and that of blood vessel trauma from 12 to 6.7 per cent. The required resistance to compressive collapse and pinching, however, obviously limits the extent to which softening an unsupported tube can be relied on to reduce trauma. In fact, occlusive kinking of a softer polyurethane catheter was noted by P. de Long and P. Kansen, as a problem occurring near connections to implanted injection ports. "A Comparison of Epidural Catheters With or Without Subcutaneous Injection Ports for Treatment of Cancer Pain," Anest. Analg. 1994, 78: 94-100. Studies have since shown a dramatic and useful further reduction of paresthesiae and blood aspiration when using even softer, coil reinforced epidural catheters.
An early radially reinforced soft-tip epidural catheter known as the THERACATH was introduced by Arrow International, Inc. ("Arrow") in 1985. It used a polyolefin heat shrink tube to encapsulate a close-wound 0.004" stainless steel spring wire with an axial safety ribbon to provide several pounds of tensile strength. The 0.004" wall polyolefin tube possessed a relatively low coefficient of friction. However, the relatively high cost attributable to the spring wire, with its double welded safety ribbon and opened distal coils largely precluded widespread use of the product.
The Arrow FLEXTIP dual-durometer epidural was introduced in 1989. It consisted of several inches of 93 Shore A polyurethane tubing, 0.5 mm (0.020") I.D. and 1.0 mm (0.040") O.D. butt welded to several feet of a stiffer polyurethane tube of identical dimensions. While the stiff proximal portion, of the epidural provided compressive strength adequate to support the compression fitting and to provide vertebral pinch resistance, the soft tip had, by virtue of its relatively thick wall, sufficient kink resistance to remain patent in the epidural space.
The Arrow FLEXTIP PLUS epidural catheter was developed in early 1992. A fabrication method of applying a continuous polyurethane coating around a relatively simple spring coil allowed many of the advantages inherent in the FLEXTIP dual-durometer epidural to be reproduced. The FLEXTIP PLUS has been widely praised by the medical profession and has been successful in the marketplace. The relatively high strength and abrasion resistance of the continuous polyurethane tube gives the catheter good tensile integrity. Nonetheless, there are some shortcomings in its design and use. Although the stainless steel coil, wound with initial tension, provides enough column strength for many anesthesiologists to insert the catheter without a stiffening stylet, it has several disadvantages. Notwithstanding the fact that the stainless steel coil provides more than adequate support against kinking and radial compression, it does not add significantly of the tensile strength to the catheter because it has no safety ribbon welded at the ends and is close-wound from 0.004" wire with a large length ratio, LR. The catheter therefore derives almost all of its tensile strength from the polyurethane coating alone. The column strength provided by the coil is judged by some to provide inadequate stiffness for catheter insertion. Furthermore, the initial tension, close-wound metal coil results in a catheter that is opaque throughout its longitudinal extent except for two "semi-transparent" flashback windows where the coil has been stretched. The presence of the metal coil also precludes use of such catheters in MRI procedures. Even though the extremely weak ferromagnetic character of the cold-worked austenitic stainless steel wire minimizes concerns over inductive heating or over forces on the catheter in magnetic field gradients, the long, continuous conductor, if allowed to lie straight along the magnet axis and against the bore of the magnet could develop high induced voltages. Though the discussion over the relative advantages of single orifice end-hole as compared to multi-orifice side-hole epidural catheters continues, there appears to be a market preference for side-hole catheters. One safety concern raised by some about side-hole epidurals involves a scenario where the catheter tip has penetrated an epidural vein so as to position the distal side-hole intravascularly while the more proximal holes remain extravascular. Low pressure injections tend to emerge from only the proximal holes, while high pressure injections flow from all openings. Given this, some postulate a hazard with side-hole catheters when aspiration and low pressure test dose does not detect a partial vascular placement, revealed suddenly later by systemic effects of a high pressure top-up dose. Inasmuch as the likelihood of vascular penetration by a soft-tip epidural is much less likely than for earlier, conventional catheters, the risk of using a soft-tip side-hole epidural can be argued to be minimal. The presence of a metal coil within the FLEXTIP PLUS, however, complicates the construction of a version where the distal end is closed and fluid is emitted through a plurality of orifices in the wall of the catheter near the distal tip. It is impossible to terminate the coil short of the side-holes, since the unsupported thin polyurethane tube would then kink or collapse and occlude. Perforation of the coating significantly reduces the strength of the catheter at that point and potentiates breakage of the tip during removal. Many anesthesiologists prefer side-hole catheters despite this and earlier mentioned risks and notwithstanding the facts that slight local tip weakness plagues all current side-hole catheters and clinical studies have shown no difference in analgesic efficacy between side-hole and end-hole catheters. They cite concern over one sided blocks caused by anesthetic jetting laterally from an end-hole catheter which is almost never oriented perfectly parallel to the spinal axis.
The cost to manufacture the Arrow FLEXTIP PLUS has been reduced to a level which allows the product to sell for only about 30% more than conventional catheters. Notwithstanding praiseworthy success in dramatically reducing paresthesiae and venous cannulation, some suggest that the FLEXTIP PLUS remains cost-ineffective. The need for improved properties at essentially no increase in cost is especially significant in the international markets where developing economies cannot bear any cost premium.
Use of helical reinforcement in catheters is far from unique in the field as evidenced by the following prior art.
U.S. Pat. No. 3,416,531 to Edwards discloses braiding a monofilament material such as metal, nylon, or teflon in opposite spiral directions into the wall of a tube in order to provide the tube with enhanced torsional stiffness without greatly increasing flexural stiffness. The presence of helical wraps in both directions, though providing torsional stiffness, requires the reinforcement to have one-half the thickness which would be possible for a non-crossing spiral wrap in only one direction. This reduces the moment of inertia of the ribbon's cross section by a factor or eight and results in a net four-fold decrease in kink resistance. Braiding is therefore a poor approach when resistance to kinking is paramount and torsional stiffness is unimportant.
U.S. Pat. Nos. 3,757,768 and 4,044,765 to Kline disclose the use of a stainless steel helical coil spring to reinforce a flexible intravenous feeding tube formed from solvent-cast polymer.
U.S. Pat. No. 3,879,516 to Wolvek teaches the use of a spiral filament of polyurethane or other solvent soluble and hence bondable material to provide integral reinforcement within a relatively thin walled catheter to provide kink-resistance. No mention is made of longitudinal strength, diameter, pitch or helix angle.
U.S. Pat. No. 3,924,632 to Cook teaches the use of fiber glass braided into the wall of a catheter to provide torsional stiffness.
U.S. Pat. No. 4,385,635 to Ruiz teaches a soft tipped angiographic catheter employing a central polyamide reinforcing tube inside a softer elastomeric sleeve. By tapering the wall thickness of the polyamide to zero within 1-7 mm of the distal end, the tip consists only of the softer urethane and is therefore itself soft.
U.S. Pat. No. 4,563,181 assigned to Mallinckrodt, Inc., discloses a soft tip intravascular catheter wherein the tip made of a nylon blend is fused or welded at a butt joint to the tubular body made of a different, stiffer nylon.
U.S. Pat. No. 4,737,153 to Shimamura et al describes a flexible, kink-resistant therapeutic tube reinforced by a spirally embedded reinforcing material. In addition to steel, stainless steel, or tungsten wire, numerous synthetic fibers, protein fibers and carbon fibers are also mentioned as candidate reinforcing materials.
U.S. Pat. No. 4,863,442 to DeMello teaches the construction of a soft tip catheter having a wire-braided teflon core and polyurethane jacket. The soft tip consists of approximately two (2) mm of polyurethane bonded to the braided core of the catheter.
U.S. Pat. No. 4,955,862 to Sepetka teaches a catheter which includes a flexible distal segment comprised of a flexible outer tube and a low-friction distal segment comprised of a flexible outer tube and a low-friction surface structure embedded in the polymer tube so as to form hard, disjoint internal surface regions. These hard surface regions are provided by the windings of a helical coil of metal or hard-surfaced polymer, and are intended to provide a low friction, small contact area to enable the catheter to be introduced over a guide wire through a tortuous path or to enable a guide wire to be passed through and manipulated within the catheter with relatively low forces.
U.S. Pat. No. 4,985,022 to Fearnot discloses a catheter having both durable proximal and flexible distal segments. The durable segment consists of a stainless steel cannula, rendered kink resistant by an external spiral wrapping with round or flat wire or by helical grooving of the outer diameter. The distal flexible segment is formed by a plastic tube surrounded by a tightly coupled wire coil. The catheter can be made with either an end hole or, by blocking the end of the wire coil and slitting the plastic tube, side exit of the medication can be allowed through the windings of the coil. Though allowing high pressure injection through a catheter of very small diameter, the complexity of the structure deems the devices far too expensive for general use.
U.S. Pat. No. 5,019,057 to Truckai discloses a flexible catheter comprising at least one tubular layer within a tubular sheath of helically crossing strands, at least one of which has a width that is four (4) to eight (8) times greater than its height and at least one of which is circular in cross section. The reinforcing strands are either mechanically retained between resilient tubular layers bound to one another, or can be coated with primer to improve their adhesion to the respective tubular layers. The braided reinforcing filaments may be made of a biaxially oriented thermoplastic material. The various embodiments of the Truckai invention are intended to exhibit increased longitudinal and torsional stiffness. However, no mention is made of longitudinal strength or specific helical angles.
U.S. Pat. No. 5,069,674 to Fearnot teaches a small diameter epidural catheter which is both flexible and kink-resistant. Positioned within the lumen of a tubular sheath (0.025-0.075 mm, 0.001-0.003" wall) of high tensile and flexural strength material, is a spirally wound coil of either hardened stainless steel (0.025-0.075 mm, 0.001-0.003" diameter), other metals or alloys, carbon filaments, hard plastic fibers or combinations thereof. A safety wire affixed to both ends, either laying within the catheter lumen or wrapped helically around the reinforcing coil, prevents stretching of the wire coil. When flexed or bent, the coils move within the lumen of the tube to distribute forces along the surface of the bent tube. Fearnot teaches a tightly wound coil at all but the distal end with a coil diameter ratio (coil diameter/wire diameter) of 4-10.
U.S. Pat. No. 5,078,702 to Pomeranz discloses a soft tip catheter having an inner sheath made of a rigid polymer. The stiffer proximal portion has a braid embedded between inner and outer sheaths, while the soft, distal tip lacks this stiffening braid. The compatibility of the inner and outer polymeric materials and the continuity of the inner sheath improves tensile integrity.
U.S. Pat. No. 5,147,315 to Weber teaches an access catheter for the female reproductive system having a segment which resists constriction by virtue of a reinforcing metal coil embedded between inner and outer tubular layers. A coil geometry suggested is 0.025 mm.times.0.125 mm (0.001".times.0.005") ribbon wound with a 0.9 mm (0.036") I.D. and a 0.1 mm (0.004") gap.
U.S. Pat. No. 5,176,660 to Truckai discloses the same invention as his '057 Patent, with the addition of longitudinally oriented wires well known in the braiding art as "zero angle" or "axial" yarns introduced through hollow horn gear shafts in the braider. The longitudinal stiffness of at least a portion of the catheter is thereby increased. No mention is made of longitudinal strength.
U.S. Pat. No. 5,178,158 to Fernando de Toledo discloses a guidewire-catheter with a soft tip. A stainless steel flat wire coil provides kink-resistance to two concentric tubes, one of which is made from polyimide extending to within a short distance of the distal tip, and one of which is heat shrunk teflon covering the entire length of the catheter and independently forming the soft, distal tip. A removable core wire allows the device to function first as a guide wire and, upon removal of the core wire, as a catheter. The polyimide sheath imparts strength to the proximal segment.
U.S. Pat. No. 5,221,255 to Mahurkar et al teaches the construction of a reinforced multiple lumen catheter employing a heavy straight reinforcing strip between two D-shaped lumens to transfer force from the proximal end of the soft silicone catheter to the distal end during insertion. It employs a spiral embedded in the catheter wall to minimize kinking. This spiral is preferably made with a thin metal wire or, alternatively, a strong polymeric monofilament.
Another "Kink-Free Spiral-Wound Catheter" is disclosed in U.S. Pat. No. 5,454,795 to Samson, wherein stiffener ribbons, typically metallic, are wound within the catheter to create a catheter having controllable stiffness. "Fibrous materials" including polyaramids (e.g. Kevlar), carbon fibers, and lower performance polymers such as Dacron and the Nylons may be used for reinforcement. Silk and cotton are acceptable natural fibers. It is taught that the preferred manner of using non-metallic ribbons is "in combination with metallic ribbons to allow `tuning` of the stiffness or as an opposite `handed` ribbon in the composite to lessen the tendency of the metallic ribbon to unwind." An inner liner may or may not be present. The outer layer is applied by heat shrinking, dipping into a molten polymer bath or into a polymer dissolved in a solid or into a suspension or latex comprising the outer cover polymer, or by spraying the material. However, no mention is made of the required longitudinal strength of the catheters and there is certainly no suggestion of the importance of helical angles or length ratios in regard to this property inasmuch as not a single diameter is specified anywhere in the Patent.
UK Patent Application GB 2,221,396A discloses an epidural catheter closed at the distal end thereof with a plurality of elongate slits or a single helical slit through the wall of the distal end of the tube so as to increase the flexibility of the tip and reduce jetting through an occluded end hole. Though mention is made of an existing wire reinforced, end-hole catheter, an object of the invention is to provide a much lower cost, "non-end-hole" catheter. No mention is made of longitudinal strength.
International Application Number PCT/US95/14865 (WO96/16690) by McWha describes "A Catheter Having Wall Support to Prevent Collapse Thereof." This Application discloses the original concept behind the "New B-D Ribflex" or "Perisafe Ribbed Lumen" epidural catheters which Becton-Dickinson has recently introduced (American Society of Anesthesiologists meeting, New Orleans, November 1996). This reference discusses the importance of a soft epidural catheter to eliminate insertion trauma. The disclosed invention proposes using a metal coil only in the proximal end of the catheter whereupon the adapter element is applied, and structuring the interior of the remaining length of the catheter with longitudinal rib elements to prevent collapsing and/or kinking during use. Such internal ribbing only prevents complete occlusion during kinking and in no way reinforces the tube against collapse.
Despite all the intentions evidenced by the above Application to produce a superior, cost-effective epidural catheter, the "New B-D Ribflex" epidural catheter actually brought to the market has no coil reinforcement anywhere, has only internal ribbing which in no way actually prevents kinking and, lacking any reinforcement, relies on the relatively high hardness of the extrusion to assure its patency. It is in essence another conventional side-hole epidural. Thus, the long-standing problem of producing a low cost, high quality soft-tip epidural catheter remains unsolved.